Continuous washing apparatus for pulp stock and the like

ABSTRACT

A washing apparatus for pulp stock or similar fibrous materials in which the stock flow is maintained solely by gravity feed. The stock is washed in an annular chamber by a recirculated counterflow of washing fluid in a plurality of washing stages. The apparatus may also contain a thickener stage for increasing stock consistency where necessary. The pulp washer may be used individually, or a plurality may be connected in series to form a system for bleach plant operations or the like. The pulp washers in such a system may also be provided with chemical injection stages for chemically treating the stock, and gravity feed of the stock is maintained in the system by decreasing the stock level over successive pulp washers to compensate for hydraulic friction.

United States Patent 91 Gunkel [451 Apr. 30, 1974 CONTINUOUS WASHING APPARATUS FOR PULP STOCK AND THE LIKE lnventorz" Kenneth M. Gunkel, 1500 Albemi St., Vancouver 5, BC, Canada Filed: June 14, 1971 Appl. No.: 153,000

Related US. Application Data Continuation of Ser. No. 862,357, Sept. 30, I969, abandoned.

US. Cl. 68/181 R, 162/232,

2lO/l89, 196; 162/232, 18, 60, 242, 251

[56] References Cited UNITED STATES PATENTS 8/l968 3,595,038 7/1971 Bergholm et aL... 68/181 R, 439,033 10/1890 Forbes 162/25] X Aremaa 68/l8l R X 7 Primary Examiner-Robert W. Jenkins Assistant Examiner-Philip R. Coe Attorney, Agent, or Firm-St. Onge Mayers and Reens [57] ABSTRACT A washing apparatus for pulp stock or similar fibrous materials in which the stock flow is maintained solely by gravity feed. The stock is washed in an annular chamber by a recirculated counterflow of washing fluid in a plurality of washing stages. The apparatus may also contain a thickener stage for increasing stock consistency where necessary. The pulp washer may be used individually, or a plurality may be connected in series to form a system for bleach plant operations or the like. The pulp washers in such a system may also beprovided with chemical injection stages for chemically treating the stock, and gravity feed of the stock is maintained in the system by decreasing the stock level over successive pulp washers to compensate for hydraulic friction.

11 Claims, 3 Drawing Figures ,1 42 l l8 0 l8 26 no no I 7 28d :23: 24d 70 22 K 2 E a: O 40 26a 2 30 4 52c M f 2%: 22

' f Z 38 a j 280 I 2: :32 .225 s: l 52b I u: a 5

22 "f; S 54a 28,, "g; 8 l Z Il K 0' I I' l 35 500 I 320 l a r PAIENIEmPa 30 I914 SHEU 1 (IF 3 By I BLAIR ass/1m AND sr. 0/va INVENTOR.

KENNETH M. GUN/(EL BY BLA/R CESAR! AND ST ONGE ATTORNEY PATENTEBAPRso r914 3L807L2-02 SHEET 3 0F 3 INVENTOR. KENNETH M GUN/(EL BY BLA/R CESARI' AND \SZ'ONGE ATTORNEYS This is a continuation of application Ser. No. 862,357 filed Sept. 30, 1969, and now abandoned.

BACKGROUND OF THE INVENTION In the processing of pulp for the'paper industry, the pulp must be washed at several stages of the processing operationffor example, after leaving the digestor and between bleaching stagesyMost modern paper mills use vacuum washers for this purpose. Vacuum washing,- however, has a number of disadvantages. The equipment is very ejxpe nsive and must be mounted a considerable distanceabove grade to provide a vacuum leg for natural drainage. Where natural drainage cannot be provided, the vacuum washing machinery requires both a vacuum pump and a liquor extraction pump which together' demand a considerable expenditure of power. More power'is'required for recirculating the liquors for dilution purposes, and for the driving of the vacuum washers themselves. Also, air entrainment generally occurs in the liquors during a vacuum washing process;

this results in a foaming condition whichnecessitates the use of additional machinery or chemicals to dispersethe fo'ami-y i v Accordingly,"representative objects of the present invention'are to provide a continuous flow diffusion pulp washer'in which the pulp is gravity fed and wash- SUMMARY OF THE INVENTION The present inventionrelates to washing apparatus for fibrous materials and more particularly to a continuous pulp washer for use, in paper making or similar processes.-

The pulp washer of the, invention is a substantially vertical structure in which pulp stock can flow from a supply and through the washer solely under the influence of gravity. In this way costly high density stock pumps can be eliminated. The stock flows through an annular pulp chamber formed flween two spaced, concentrically mounted perforated-plate cylinders. The outer perforated-plate cylinder is surrounded by a spaced jacket and the space-between the jacltet and the outer perforated-plate cylinder is divided into a plurality of vertically stacked outer compartments. Similarly the inner perforated-plate cylinder is divided into a corresponding plurality of inner compartments. The corresponding inner and outer compartments are thus in fluid communication through adjacent portions of the two perforated-platecylinders, and comprise individual washing stages. i I

Fluid circulating means are provided which cause washing fluid to flow between each corresponding pair of inner and outer compartments in each washing stage. The fluid thus washes through the pulp stock in the pulp chamber and removes the liquor therein. Preferably, the flow of washing fluid comprises a recirculated 'counterflow, that is, the washing fluid exiting from each lower washing stage is recirculated through the next successively higher stage and so on. In this way the pulp stock is washed several times and treated with increasingly purer wash fluid at each successively lower washing stage in order to obtain an optimum removal of liquor and/or contaminants.

The pulp washer may also be provided with a thickening stage adjacent the entrance for the pulp stock. The thickening stage is essentially identical to the washing stages but has a separtate circulating system which removes liquor from the pulp stock entering the washer.-This brings the pulp stock to a desired thickened consistency for maximizing the output of the washer.

The pulp washermay be used individually, or a plurality of them may be connected in series to produce a pulp washer system for use in, for example, a bleaching operation; At least some of the pulp washers in such a system are typically provided with chemical injection stages; these stages are again similar to the washing stages but are generally positioned near the outlet of the pulp washer. Each chemical injection stage or stages is also provided with a separate circulating means to circulate a chemical reagent in solution through the pulp stock as it passes the chemical injection stage in muchthe same way the pulp stock is washed in the washing stages. Further, the fluid head or level of pulp stock over each successive washer in such a system is lowered by an amount at least sufficient to overcome hydraulic friction in the system. In this way the pulp stock is gravity fed through the system without need for expensive stock pumps.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3 is a diagrammatic front elevation view in section showing a plurality of the diffusion pulp washers of the invention connected together in series for a bleach plant operation or the like.

Similar reference characters refer to similar parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED I EMBODIMENTS Referring now to FIGS. 1 and 2, the continuous pulp washer of the invention is shown in a typical installation for plants having conventional digestion equipment. It

will be understood, however, that the pulp washer can be used in other environments and for more specialized applications. As shown in FIG. 1, pulp washer 10 is mounted below a conical-bottom receiver 12 which holds paper stock received, for example, from a continuous digestor or from the blow tank of a batch digestor. Receiver 12 may be supported in its elevated position over pulp washer by a plurality of vertical support beams 11. Typically receiver 12 is provided with a conventional side-entrance agitator to mix the stock and maintain homogeneity, and with a conventional stock level control, neither of which is shown in FIG. 1 for purposes of clarity. Receiver 12 may also be provided with a consistency control to regulate stock consistency.

An outer perforated-plate cylinder 14 is suspended verticallyor substantially vertically below receiver 12 and preferably attached thereto by flush head rivots or screws 15 (FIG. ,2) passing through the plate flanges l6 and the abutting receiver flanges 18. Outer perforatedplate cylinder 14 which will be described in greater detail hereinafter is surrounded by a spaced, vertically disposed cylindrical jacket 20. A plurality of annular, spaced baffles 22 disposed in substantially horizontal planes secure jacket to outer perforated-plate cylinder l4. Baffles 22 may be connected both to jacket 20 and to outer perforated-plate cylinder 14 by flush head rivots, screws, welding or the like. As can be seen in FIG. 1, spaced baffles 22 also divide the annular space or jacket chamber between jacket 20 and outer perforated-plate cylinder 14 into-a plurality of vertically stacked, annular outer compartments 24a, 24b, 24c and 24d.

An inner perforated-plate cylinder 26 is mounted concentrically within outer perforated-plate cylinder 14. Inner perforated-plate cylinder 26 is divided into a corresponding plurality of vertically stacked inner compartments 28a, 28b, 28c and 28d by a plurality of spaced baffle discs 30. Baffle discs 30 have a diameter substantially equal to the inner diameter of inner perforated-plate cylinder 26 and are secured in substantiments. The perforations should be small enough to prevent the p'assageof the fibers in the pulp stock; however, the number and spacing of the perforations in both plates are preferably such that the percentage of open area is as large as possible consistent with the structural strength of the perforated-plate cylinders.

This minimizes the flow velocity of the washing or treating fluid and keeps it in an approximate laminar flow condition.

Still referring to FIG. 1, inner perforated-plate cylinder 26 is supported in its concentric position within outer perforated-plate cylinder 14 by four substantially vertical pipes 32a, 32b, 32c and 32d; these are preferably equally spaced within the periphery of inner perforated-plate cylinder 26 as shown in FIG. 2 Pipes 32a-d pass through and are secured along their lengths to each baffle disc 30, and they are also rigidly secured at their lower ends'in a bottom closure plate 34 (FIG. 1). Closure plate 34 is securely attached to the bottom flange 36 of outer perforated-plate cylinder 14. As shown in FIG. 1, each pipe 32a-d is provided respec- 4 tively with an opening 38a, 38b, 38c and 38d into one corresponding inner compartment'28ad, but has no openings to the-other inner compartments. Each opening 38a-d may thus function as either an inlet or outlet to one inner compartment as will be more fully dis cussed hereinbelow.

Still referring to FIG. 1, it can be seen that the concentrically spaced mounting of inner perforated-plate cylinder 26 and outer perforated-plate cylinder 14 definesan annular chamber 40. Chamber 40, called an annular pulp chamber herein, extends vertically through pulp washer 10 and has an entrance 42 opening directly into receiver 12. Accordingly, pulp stock flowing under the influence of gravity from receiver 12 flows directly through entrance 42 into annular chamber 40 and thus through pulp washer 10. It is during the passage of the pulp stock through pulp chamber 40 that it is'washed or treated in accordance with the invent- An'inverted cone deflector 44 is preferably mounted to the upper end of inner perforated-plate'cylinder 26 so as to extend into the interior of receiver 12 as shown in FIG. 1. Inverted cone deflector 44 thus serves to direct and facilitate the flow of pulp stock from receiver 12 into entrance 42 of annular pulp chamber 40.

With reference to.FIG. 1, it can be seen that by the [unique] mounting of inner perforated-plate cylinder 26 on pipes 32a-d, obstructions to thefree flow of pulp stock through the pulp washer are avoided, particularly in the critical area of entrance 42 and in pulp chamber The thickness of annular pulp chamber 40, which is measured by the spacing between the outer and inner perforated-plate cylinders 14 and 26, should be maintained relatively small; accordingly only a relatively srnallvolume of pulp stock is exposed for treatment at any given point in the apparatus during operation. This permits the thorough washing and/or treatment of the pulp stock with a substantially greater volume of washing or treating fluid. Typically, the thickness t of annular pulp chamber 40 as shown in FIG. 1 will be maintained at about one-twelfth the diameter of inner perforated-plate cylinder 26.

The washing of the, pulp stock flowing through annular pulp chamber 40is preferably accomplished by a recirculated counterflow of washing fluid such as water. The system 45 for circulating the'washing fluid is shown in FIGS. 1 and 2; pipes 32a, 32b and 320 are re- 'spectively connected by means of lengths of tubing 46a, 46b and 46c to the outlets of three separate fluid pumps 48a, 48b and 480 (FIG. 2). As will be seen,the pumps constitute the only moving parts in the pulp washing apparatus. The inlet ofpump 48a is connected by an inlet line 50a to a source of the washing fluid (not shown). Pereferably inlet line 50a will also be provided with a flow meter so that the flow of washing fluid can be regulated in accordance with the flow of pulp stock. In the case of water, the washing fluid should be heated to as close to 212F as possible to lower the viscosity of the waste liquors and make them easier to displace from the pulp stock. Accordingly, insulation should be provided on the pulp washer where appropriate to reduce heat losses.

As can be seen in FIG. 1, washing fluid is first circulated by pump 48a into pipe 32a and then through opening 38a to inner compartment 28a. As the washing fluid fills inner compartment 28a, it flows through the perforations in inner perforated-plate cylinder 26. The

washing fluid then flows through the pulp stock which in turn is flowing downward in the portion of annular pulp chamber 40 surrounding compartment 28a; it

then flows through the perforations in outer perforated-plate cylinder 14 and into outer compartment 24a. The flow of washing fluid like the flow of pulp stock should be approxiamately laminar for optimum results.

The adjacent pair of inner and outer compartments 28a and 24a may thus be considered as forming one washing stage for the pulp stock. As will be seen from the following discussion, the two succeedingly higher compartment pairs 28b, 24b and 28c, 240 also functio as similar washing stages.

To maintain washing fluid circulation, an outlet 52a is provided in outer compartment 24a. Outlet 52a is connected by appropriate tubing 54a to the inlet of the next successive pump 48b. as shown in FIG. 2. Pump 48b then recirculates the washing fluid from the first washing stage (now contaminated with weak liquors from the pulp stock) back through tubing 46b into the second pipe 32b (FIG. 1) and out opening 38b into the next successivelyhigher washing stage which comprises inner compartment 28b and outer compartment 24b. Therein a pulp stock washing operation occurs in the same manner as previously discussed with reference to the lower washing stage.

' As shown in FIGS. 1 and 2, the two consecutively higher outer compartments 24b and 24c are provided with corresponding outlets. 52b and 52c connected respectively by lengths of tubing 54b and 54c to the inlets of successive pumps 48b and 480. Thus increasingly contaminated washing fluid is circulated from the sec.- ond washing stage to the third and from the third out of the apparatus. The outlet of pump 480 is connected by a drain pipe 54c to a waste tank, or in some instalthe inner to the outer compartments, the flow may be reversed with 'equivalentresults. The flow should however be in the same direction in all thewashing stages to prevent turbulence between stages.

It can thus be seen with reference to FIG. 1 that the washing fluid is recirculated through the pulp washer from each lower washing stage to the next consecutively higher washing stage. The washing fluid accordingly becomes increasingly contaminated with the waste products from the pulp stock unitil it is discharged as a strong liquor effluent to a waste tank'or the like.

Conversely, the pulp stock flowing down through annular pulp chamber 40 is washed at each successively lower washing stage by less contaminated washing fluid, until it reaches the lowermost washing stage for treatment with essentially pure washing fluid. Moreover, the volume of washing fluid at each stage is also considerably greater than the volume of pulp stock being washed. In this way a relatively high degree of purity can be obtained in the pulp issuing from the pulp chamber below the lowest washing stage.

The washed pulp stock then flows into a discharge chamber 56 above closure plate 34 (FIG. 1) and from there outthrough a discharge 58 onto a discharge line 60. Discharge 58 is'preferably centered in closure plate 34 as shown to make for complete symmetry in the pulp washer and facilitate the even flow of stock. Line 60 typically carriesthe pulp to a stock chest (not shown) in which the fluid level is maintained below the levelinreceiver 12 so that the hydrostatic head will move the stock through the apparatus under the force of gravity alone. Discharge line 60 is'also preferably provided with a flow control valve having a magnetic .flow meter to maintain the fluid flow at a predetermined rate.

lations to a system which adds the waste washing fluid y} to the receiver 12 for purposes of diluting the consistency of the pulp stock therein.

In operation, all the washing stages are completely flooded with washing fluid. Thus there is no possibility of air entrainment and resultant undesirable foaming.

propriate metering system. Preferably and more 'sim-' ply, however, equal flows are maintained by using pumps 48a, 48b, 48c and 48c which are identical, positive displacement types. These identical pumps are simultaneously driven through a common, variable speed drive 53 (FIG. 2) by means of a single motor 55 to achieve equal flow rates in each of the three washing stages shown. Moreover, little power is required to drive such pumps since their inlet and outlet pressures are substantially the same; thus the pumps serve only as positive displacement circulators.

It will be understood that although three washing stages have been shown for purposes of illustration, more or fewer stages may be used for a particualar application without departing from the invention. Also, while the flow of washing fluid has been shown from Aseparate circulation system 62 is provided for the thickening stage as shown in'FIG. 2. The inlet of one fluid pump 64 is connected by tubing 66 to pipe 32d which has an opening 38d into inner compartment 28d (FIG. 1). The inlet of an adjacent pump 68 is connected by tubing 70 to the outlet 72 from outer compartment 24d. The outlet of each pump is connected to a drain pipe 74 which may be common to both pumpsas shown. w h p j 7 As shown by the arrows in FIG. 1, in operation pumps 64 and 68 respectively draw off fluid simultaneously from inner compartment 28d and outer compartment 24d. Therate of fluid withdrawal is adjusted to produce the desired thickened consistency for the pulp stock flowing in chamber .40. Preferably, pumps 64 and 68 operate at the same rate; this may be accomplished by the use of identical, positive displacement pumps connected to a common, variable speed drive .76 driven p be washed directly as it flows from receiver 12, or it 7 may be diluted in the receiver as previously discussed. Accordingly, the thickening stage shown in FIG. 1 may be eliminated or converted to an additional washing stage for some applications.

The pulp washer of the invention, in addition to the single unit application previously discussed, also lends itselfideally to applications requiring multiple units connected in series. One such representative application is in a bleach plant operation.

Referring now to FIG. 3, there are shown three pulp washers connected together in series for use in such a bleach plant or similar operation. The first pulp washer in the series 10a is suspended under a conical-bottom receiver 12a in the manner'previously discussed. Re-

ceiver 12a is typically preceded in the system by a standard chlorine injector and mixer, and a series of relatively small bleaching towers all connected together to provide alternate up-flow and down-flow of the pulp stock. The time it takes the pulp stock to flow through .the series of bleaching towers provides the needed reaction time for the quantity of pulp being bleached.

The last tower in the'chlorine stage empties into receiver 12a, and the pulp stock which enters therein washed by a recirculated counterflow of washing fluid in the manner previously discussed.

From bottom washing stage a,, the pulp stock may, if necessary, pass through an additional stagea located just above discharge chamber 56 and discharge 58. Stage a comprises a chemical injection stage wherein the washed pulp stock may be treated with one or more chemical reagents. The reagents are normally provided in a heated solution to speed up the reaction. The provision of sucha chemical injection stage permits elimination of the separate chemical mixing and heating devices used in prior art washing systems, particularly in vacuum washing systems. I

Chemical injection stage a is provided withv a separate fluid circulation system; this chemical injection circulation system may be identical with the system shown for one washing stage in FIG. 2, except that the inlet of the first fluid pump is connected to a source of chemical reagent in solution, and the outlet of the second fluid pump is connected to a drain pipe rather than to the inlet of a successively higher stage. The drained reagents may then be recirculated to a chemical mixing tank for restrengthening. Thus, the pulp stock passing through pulp chamber 40 adjacent stage a is treated with a chemical reagent in substantially the same manner as it is treated with washing fluid in the three upper stages a a I From chemical injection stage a the pulp stock continues downward, still under the influence of gravity, into discharge chamber 56 and out through discharge 58; from there it flows through discharge line 60 to the entrance pipe 82 of the next receiver 12d in the series. While discharge line 60 and receiver 12d provide only a single up-flow and down-flow between pulp washer a and 10b, it will be understood that further convent- 8 ional up-flow, down-flow towers may be interposed where conditions require.

Preferably a gravity feed is maintained for the pulp stock between each of the pulp washer units in a system such as shown in FIG. 3. By maintaining such a gravity feed, costly, high density stock pumps which would otherwise be required between separate units in the system can be eliminated. For this purpose, the fluid head (which comprises the stock level in receiver 12b) is maintained at a lower level than the stock level in the preceding receiver 12a. This difference in level which is shown by d in FIG. 3 should be more sufficient to compensate for hydraulic friction in the system.

The fluid treatment stages b,, b b b in the second pulp washer 10d may comprise any desired number and combination of washing and/or chemical injection stages. Typically, a thickening stage is not required in unit 10b because of the previous thickening of the pulp stock in stage a 'of pulp washer 10a.

From pulp washer 1012 the pulp stock then flows up mically treated at stages C C C C and then out through discharge 58 for storage or subsequent processing. 4 i I Although a three unit system has been shown for illustrative purposes in FIG. 3, it will be understood that more or fewer units may be connected together in series as the particular application and conditions require.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently' attained and, since certain changes may be made in the above constructions wit hout departing from the scope of the invention, it is intended thatall matter contained in the above description or shown in the accompanying drawings shall be interpreted asillustrativeand not in a limiting sense.

Having described 'my invention, what I claim as new and desire to secure by Letters Patentis:

1. A continuous pulp washer comprising, in combina- A. a substantially vertically disposed jacket,

B. an outer perforated-plate cylinder mounted concentrically within said jacket and spaced therefrom to form an annular jacket chamber,

1. said outer perforated-plate cylinder defining an entrance for pulp stock adjacent the upper end and a discharge for pulp stock adjacent the lower end thereof,

C. an inner perforatedplate cylinder mounted concentrically within said outer perforated-plate cylinder and spaced therefrom to form an annular pulp chamber communicating .with said entrance adjacent the upper end and with said discharge adjacent the lower end thereof,

D. means separating said annular jacket chamber stacked inner compartments each corresponding in position to one said outer compartment,

l. with the pairs of said corresponding inner and outer compartments being in fluid communication through adjacent portions of said inner and outer perforated-plate cylinders, and

F. fluid circulating means associated with each pair of corresponding inner and outer compartments which provide fluid flow between each pair of corresponding compartments through adjacent portions of said inner and outer perforated-plate cylinders, whereby the pulp stock flowing in said annular pulp chamber between said portions is treated by said fluid.

2. A continuous pulp washer as defined in claim 1 wherein at least some pairs of said corresponding inner and outer compartments comprise washing stages, and wherein the outlet of each washing stage communicates with the inlet of the next successively higher washing stage whereby a recirculated counterflow of washing fluid passes through said pulp washer causing said pulp stock to be washed by decreasingly contaminated washing fluid as it flows through said annular pulp chamber past successive washing stages.

3. A continuous pulp washer as defined in claim 2 wherein said fluid circulating means for said washing stages comprise a plurality of identical positive displacement pumps driven from a common drive, one

said pump connecting the inlet of the lowermost washing stage to a source of washing fluid, another said pump connecting the outlet of the uppermost washing stage to a drain, and one or more intermediate pumps connected in seriatum between the outlet of each lower and the inlet of each successively higher washing stage, whereby the flow of washing fluid through all washing stages is maintained at an equal rate.

4. A continuous pulp washer as defined in claim 1 in cluding an inverted cone deflector mounted on the upper end of said inner perforated-plate cylinder to direct and facilitate the flow of pump stock from a receptacle mounted over said entrance into said annular pulp chamber.

.5. A continuous pulp washer as defined in claim 1 and further including a pair of said corresponding inner and outer compartments adjacent said entrance which comprise a thickening stage, and separate fluid circulating means associated with said thickening-stage for draining liquor from said pulp as it flows through said annular pulp chamber in said thickening stage, whereby a predetermined thickened pulp consistency is achieved.

6. A continuous pulp washer as defined in claim 1 and further including a pair of said corresponding inner and outer compartments adjacent said discharge which comprise a chemical injection zone, and means for chemically treating said pulp as it flows through said annular pulp chamber in said chemical injection zone.

7. A continuous pulp washer as defined in claim '1 wherein said outer compartments are formed by a plurality of annular, spaced baffles secured in substantially horizontal planes between said jacket and said outer perforated-plate cylinder, and said inner compartments are formed by a corresponding plurality of spaced baffle discs of a diameter substantially equal to the inner diameter of said inner perforated-plate cylinder,

said discs being supported in substantially horizontal planes within said inner perforated-plate cylinder by a plurality of substantially vertical pipes extending the end and a discharge for pulp stock adjacent the lower end thereof,

3. an inner perforated-plate cylinder mounted concentrically within said outer perforated-plate cylinder and spaced therefrom to form an annular pulp chamber communicating with said entrance adjacent the upper end and with said dis- I charge adjacent thelower end thereof,

4. means separating said annular jacket chamber into a plurality of vertically stacked outer compartments, i

5. means separating the interior of said inner perforated-plate cylinder into a plurality of vertically stacked inner compartments each corresponding in position to one said outer compartment,

a. with the pairs of said corresponding inner and outer compartments being in fluid communication through adjacent portions of said inner and outer perforated-plate cylinders, and

6. fluid circulating means associated with each pair of corresponding inner and outer compartments for providing fluid flow between each pair of corresponding compartments through the adjacent portions of said inner and outer perforated-plate cylinders, whereby the pulp stock flowing in said annular pulp chamber between said portions is treated by said fluid,

7. means connecting said plurality of pulp washers in seriatim with the entrance to the first said pulp washer in the series in communication with a pulp supply, each intermediate pulp wahser in the series having its entrance in communication with the pulp discharge of the previous pulp washer and its discharge in communication with the entrance of the next successive pulp washer, and with a final pulp discharge on the last said pulp washer,

8. means maintaining the fluid head of pulp stock over each successive pulp washer in the series at a lower level than the fluid head over the previous pulp washer in an amount to more than compensate for hydraulic friction in said system and thereby permit gravity feed of pulp stock through said system.

9. A continuous pulp washer system as defined in claim 8 wherein at least one of the pulp washers in the series contains a chemical injection zone comprising a pair of said corresponding inner and outer compartments adjacent the discharge thereof, and means for chemically treating said pulp as it flows through said annular pulp chamber in said chemical injection zone.

10. A continuous pulp washer as defined in claim 1 wherein the thickness of said annular pulp chamber is about one-twelfth the diameter of said inner perforated-plate cylinder.

11. A continuous pulp washer comprising in combination A. a jacket having an entrance for pulp stock adjacent one end and a discharge for pulp Stockadjacent the opposite end thereof;

B. means forming a relatively narrow pulp chamber within said jacket and defined by walls pervious to the washing fluid,

1. said pulp chamber communicating at one end with said entrance and at the opposite end with the discharge;

C. means forming a plurality of jacket chambers substantially contiguous with said pulp chamber and in said portions is treated by said fluid. 

1. A continuous pulp washer comprising, in combination: A. a substantially vertically disposed jacket, B. an outer perforated-plate cylinder mounted concentrically within said jacket and spaced therefrom to form an annular jacket chamber,
 1. said outer perforated-plate cylinder defining an entrance for pulp stock adjacent the upper end and a discharge for pulp stock adjacent the lower end thereof, C. an inner perforated-plate cylinder mounted concentrically within said outer perforated-plate cylinder and spaced therefrom to form an annular pulp chamber communicating with said entrance adjacent the upper end and with said discharge adjacent the lower end thereof, D. means separating said annular jacket chamber into a plurality of vertically stacked outer compartments, E. means separating the interior of said inner perforated-plate cylinder into a plurality of vertically stacked inner compartments each corresponding in position to one said outer compartment,
 1. with the pairs of said corresponding inner and outer compartments being in fluid communication through adjacent portions of said inner and outer perforated-plate cylinders, and F. fluid circulating means associated with each pair of corresponding inner and outer compartments which provide fluid flow between each pair of corresponding compartments through adjacent portions of said inner and outer perforated-plate cylinders, whereby the pulp stock flowing in said annular pulp chamber between said portions is treated by said fluid.
 2. an outer perforated-plate cylinder mounted concentrically within said jacket and spaced therefrom to form an annular jacket chamber, a. said outer perforated-plate cylinder defining an entrance for pulp stock adjacent the upper end and a discharge for pulp stock adjacent the lower end thereof,
 2. A continuous pulp washer as defined in claim 1 wherein at least some pairs of said corresponding inner and outer compartments comprise washing stages, and wherein the outlet of each washing stage communicates with the inlet of the next successively higher washing stage whereby a recirculated counterflow of washing fluid passes through said pulp washer causing said pulp stock to be washed by decreasingly contaminated washing fluid as it flows through said annular pulp chamber past successive washing stages.
 3. A continuous pulp washer as defined in claim 2 wherein said fluid circulating means for said washing stages comprise a plurality of identical positive displacement pumps driven from a common drive, one said pump connecting the inlet of the lowermost washing stage to a source of washing fluid, another said pump connecting the outlet of the uppermost washing stage to a drain, and one or more intermediate pumps connected in seriatum between the outlet of each lower and the inlet of each successively higher washing stage, whereby the flow of washing fluid through all washing stages is maintained at an equal rate.
 3. an inner perforated-plate cylinder mounted concentrically within said outer perforated-plate cylinder and spaced therefrom to form an annular pulp chamber communicating with said entrance adjacent the upper end and with said discharge adjacent the lower end thereof,
 4. means separating said annular jacket chamber into a plurality of vertically stacked outer compartments,
 4. A continuous pulp washer as defined in claim 1 including an inverted cone deflector mounted on the upper end of said inner perforated-plate cylinder to direct and facilitate the flow of pump stock from a receptacle mounted over said entrance into said annular pulp chamber.
 5. A continuous pulp washer as defined in claim 1 and further including a pair of said corresponding inner and outer compartments adjacent said entrance which comprise a thickening stage, and separate fluid circulating means associated with said thickening stage for draining liquor from said pulp as it flows through said annular pulp chamber in said thickening stage, whereby a predetermined thickened pulp consistency is achieved.
 5. means separating the interior of said inner perforated-plate cylinder into a plurality of vertically stacked inner compartments each corresponding in position to one said outer compartment, a. with the pairs of said corresponding inner and outer compartments being in fluid communication through adjacent portions of said inner and outer perforated-plate cylinders, and
 6. fluid circulating means associated with each pair of corresponding inner and outer compartments for providing fluid flow between each pair of corresponding compartments through the adjacent portions of said inner and outer perforated-plate cylinders, whereby the pulp stock flowing in said annular pulp chamber between said portions is treated by said fluid,
 6. A continuous pulp washer as defined in claim 1 and further including a pair of said corresponding inner and outer compartments adjacent said discharge which comprise a chemical injection zone, and means for chemically treating said pulp as it flows through said annular pulp chamber in said chemical injection zone.
 7. A continuous pulp washer as defined in claim 1 wherein said outer compartments are formed by a plurality of annular, spaced baffles secured in substantially horizontal planes between sAid jacket and said outer perforated-plate cylinder, and said inner compartments are formed by a corresponding plurality of spaced baffle discs of a diameter substantially equal to the inner diameter of said inner perforated-plate cylinder, said discs being supported in substantially horizontal planes within said inner perforated-plate cylinder by a plurality of substantially vertical pipes extending the length of said inner perforated-plate cylinder and through said baffle discs, each said pipe having an opening into one said inner compartment for fluid communication therewith.
 7. means connecting said plurality of pulp washers in seriatim with the entrance to the first said pulp washer in the series in communication with a pulp supply, each intermediate pulp wahser in the series having its entrance in communication with the pulp discharge of the previous pulp washer and its discharge in communication with the entrance of the next successive pulp washer, and with a final pulp discharge on the last said pulp washer,
 8. means maintaining the fluid head of pulp stock over each successive pulp washer in the series at a lower level than the fluid head over the previous pulp washer in an amount to more than compensate for hydraulic friction in said system and thereby permit gravity feed of pulp stock through said system.
 8. A gravity feed continuous pulp washer system comprising, in combination: A. a plurality of continuous diffusion pulp washers each comprising,
 9. A continuous pulp washer system as defined in claim 8 wherein at least one of the pulp washers in the series contains a chemical injection zone comprising a pair of said corresponding inner and outer compartments adjacent the discharge thereof, and means for chemically treating said pulp as it flows through said annular pulp chamber in said chemical injection zone.
 10. A continuous pulp washer as defined in claim 1 wherein the thickness of said annular pulp chamber is about one-twelfth the diameter of said inner perforated-plate cylinder.
 11. A continuous pulp washer comprising in combination A. a jacket having an entrance for pulp stock adjacent one end and a discharge for pulp stock adjacent the opposite end thereof; B. means forming a relatively narrow pulp chamber within said jacket and defined by walls pervious to the washing fluid, 